OFDM signal transmission method and apparatus

ABSTRACT

An orthogonal frequency division multiplexing (OFDM) signal transmission apparatus which transmits OFDM signals by using a plurality of transmission antennas includes a subcarrier setting device which sets signals for subcarriers so as to use some of the subcarriers of the OFDM signals as pilot subcarriers to transmit pilot signals and use the remaining subcarriers as data subcarriers to transmit data signals, the subcarrier setting device changing polarities of signals for the pilot subcarriers for each transmission antenna.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 12/604,712, filed Oct.23, 2009, which is a continuation of Ser. No. 11/076,051, filed Mar. 10,2005, now U.S. Pat. No. 7,742,533 and is based upon and claims thebenefit of priority from prior Japanese Patent Applications No.2004-071732, filed Mar. 12, 2004, No. 2004-176096, filed Jun. 14, 2004,No. 2004-235349, filed Aug. 12, 2004, No. 2004-256247, filed Sep. 2,2004; and No. 2005-052949, filed Feb. 28, 2005, the entire contents ofall of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an orthogonal frequency divisionmultiplexing (OFDM) signal transmission method and apparatus using aplurality of transmission antennas.

2. Description of the Related Art

Of OFDM signal transmission apparatuses, especially, a system whichtransmits different data simultaneously through a plurality oftransmission antennas can transmit data at high transmission. On theother hand, the data error rate characteristic readily degrades. Amethod is known, in which on the transmitter side, a signal known toreceiver, called a pilot symbol is superposed on one or a plurality ofspecific subcarriers to form pilot subcarriers. On the receiver side,channel equalization or frequency offset compensation is done for eachsubcarrier on the basis of the pilot subcarriers, thereby obtaining adecoded signal with an improved data error rate performance.

When a single known signal is transmitted from a plurality oftransmission antennas by using pilot subcarriers of the same frequency,as described above, the transmission signals of the respective pilotsubcarriers interfere with each other to form directional beams. In theIEEE802.11a standard, there are 52 subcarrier populated and four of themare assigned as pilot subcarrier. However, the directional beam of allthe pilot subcarriers are directed in almost the same direction becausethe interval (about 4.4 MHz) between the pilot subcarriers is smallerthan the carrier frequency (5 GHz). In this case, the null points atwhich the electric fields of the respective directional beams abruptlydecrease are also directed in the same direction. For this reason, it isalmost impossible to receive the pilot subcarrier in the direction ofthe null points, and the reception performance drastically degrades.

To cope with this problem, Jpn. Pat. Appln. KOKAI Publication No.2003-304216 discloses a technique, in which a pilot subcarrier istransmitted from only one transmission antenna, and null signals in thefrequency band of the pilot subcarrier are transmitted from theremaining transmission antennas. According to this technique, theproblem of mutual interference between pilot subcarriers, which occurswhen pilot subcarriers are transmitted from a plurality of transmissionantennas, is avoided. Hence, any degradation in reception performancedue to directional beam formation can be prevented.

In the method of transmitting a pilot subcarrier from a singletransmission antenna, like Jpn. Pat. Appln. KOKAI Publication No.2003-304216, the total transmission power of the pilot subcarrier is lowas compared to a system which transmits pilot subcarriers from aplurality of transmission antennas. This degrades the receptionperformance of the receiver.

When the transmission power of the pilot subcarrier from the singletransmission antenna is made higher than that of the data subcarrierfrom each transmission antenna, the total transmission power of thepilot subcarrier can be increased, and the reception performanceimproves. However, when the transmission power of the pilot subcarrierfrom the single transmission antenna is increased, a variation intransmission power occurs in the frequency band of the OFDM signal. Thismay result in composite triple beat (CTB) or increase the dynamic rangeof the transmission signal to make the specifications (especially inputdynamic range) of a digital to analog (D/A) converter of the receiverstrict.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide an OFDM signaltransmission apparatus which decreases the composite triple beat withoutdecreasing the transmission power of a pilot subcarrier. It is anotherobject of the present invention to increase the area where high-qualityreception is possible.

The first aspect of the present invention provides an orthogonalfrequency division multiplexing (OFDM) signal transmission apparatuswhich transmits OFDM signals by using a plurality of transmissionantennas, comprising: a subcarrier setting device configured to set someof subcarriers of the OFDM signals to pilot subcarriers for transmittingpilot signals, and remaining subcarriers thereof to data subcarriers fortransmitting a data signal, polarities of the pilot subcarriersdiffering among the transmission antennas.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram of an OFDM communication system according toan embodiment of the present invention;

FIGS. 2A and 2B are views showing the subcarrier location in OFDMsignals transmitted from two transmission antennas according to thefirst embodiment of the present invention;

FIG. 3 is a block diagram of an OFDM signal transmission apparatus shownin FIG. 1;

FIG. 4 is a block diagram of a pilot subcarrier insertion unit accordingto the first embodiment of the present invention;

FIG. 5 is a block diagram of an OFDM signal reception apparatus shown inFIG. 1;

FIG. 6 is a view for explaining the residual phase error detectionprinciple according to the first embodiment of the present invention;

FIG. 7 schematically shows the beam patterns of each of first and secondtransmission antennas and the beam patterns combined with the first andsecond transmission antennas when pilot subcarriers having the samepolarity pattern are transmitted from the first and second transmissionantennas.

FIG. 8 schematically shows the beam patterns of each of first and secondtransmission antennas and the beam patterns combined with the first andsecond transmission antennas when the polarity pattern of the firstpilot subcarriers transmitted from the first transmission antenna andthat of the second pilot subcarriers transmitted from the secondtransmission antenna are different according to the first embodiment;

FIG. 9 is a graph showing the average normalized reception level ofpilot subcarriers in the reception apparatus when the pilot subcarrieris transmitted by using the embodiment of the present invention;

FIG. 10 is a graph showing the reception power when first and secondpilot subcarriers are transmitted by using certain polarity data;

FIGS. 11A to 11D are views showing the polarity patterns of pilotsubcarriers transmitted from the respective transmission antennas whenthe first embodiment of the present invention is extended to an OFDMapparatus having four transmission antennas;

FIG. 12 is a block diagram of a pilot subcarrier insertion unitaccording to the second embodiment of the present invention;

FIG. 13 is a view showing various polarity patterns of pilot subcarrierstransmitted from the respective transmission antennas according to thesecond embodiment of the present invention;

FIG. 14 is a block diagram of a pilot subcarrier insertion unitaccording to the third embodiment of the present invention;

FIG. 15 is a view showing an example of the structure of a wirelesspacket of an OFDM signal;

FIG. 16 is a block diagram of a pilot subcarrier insertion unitaccording to the fourth embodiment of the present invention;

FIGS. 17A and 17B are views showing the subcarrier location in OFDMsignals transmitted from two transmission antennas according to thefourth embodiment of the present invention;

FIG. 18 is a view for explaining the residual phase error detectionprinciple according to the fourth embodiment of the present invention;

FIG. 19 is a block diagram of a pilot subcarrier insertion unitaccording to the fifth embodiment of the present invention;

FIGS. 20A and 20B are views showing the subcarrier location in OFDMsignals transmitted from two transmission antennas according to thefifth embodiment of the present invention;

FIG. 21 is a block diagram of a pilot subcarrier insertion unitaccording to the sixth embodiment of the present invention;

FIGS. 22A and 22B are views showing the subcarrier location in OFDMsignals transmitted from two transmission antennas according to thesixth embodiment of the present invention;

FIGS. 23A and 23B are views for explaining the residual phase errordetection principle according to the sixth embodiment of the presentinvention;

FIG. 24 is a block diagram of a pilot subcarrier insertion unitaccording to the seventh embodiment of the present invention;

FIGS. 25A and 25B are views showing the subcarrier location in OFDMsignals transmitted from two transmission antennas according to theseventh embodiment of the present invention;

FIG. 26 is a view showing an example of a wireless communicationpreamble signal;

FIG. 27 is a view showing a wireless communication preamble signal basedon the IEEE 802.11a standard;

FIG. 28 is a block diagram of an OFDM signal reception apparatus whichreceives the wireless packet shown in FIG. 26;

FIG. 29 is a view for explaining processing executed by a residual phaseerror detector according to the eighth embodiment of the presentinvention;

FIGS. 30A and 30B are views showing the subcarrier location in OFDMsignals transmitted from two transmission antennas according to a priorart;

FIG. 31 is a block diagram of an OFDM signal transmission apparatusaccording to the ninth embodiment of the present invention;

FIG. 32 is a view showing the pilot subcarrier location in an OFDMsignal transmitted from the first transmission antenna according to the10th embodiment of the present invention;

FIG. 33 is a view showing the pilot subcarrier location in an OFDMsignal transmitted from the second transmission antenna according to the10th embodiment of the present invention;

FIG. 34 is a view showing the pilot subcarrier location in an OFDMsignal transmitted from the third transmission antenna according to the10th embodiment of the present invention;

FIG. 35 is a view showing the pilot subcarrier location in an OFDMsignal transmitted from the fourth transmission antenna according to the10th embodiment of the present invention;

FIG. 36 is a view showing an example of polarity data of the pilotsubcarrier according to the 10th embodiment of the present invention;

FIG. 37 is a view showing another example of polarity data of the pilotsubcarrier according to the 10th embodiment of the present invention;

FIG. 38 is a block diagram of a pilot subcarrier insertion unitaccording to the 10th embodiment of the present invention;

FIG. 39 is a block diagram of an OFDM signal reception apparatusaccording to the 10th embodiment of the present invention;

FIG. 40 is a block diagram of a modification of the OFDM signalreception apparatus according to the 10th embodiment of the presentinvention; and

FIG. 41 is a block diagram of a modification of the pilot subcarrierinsertion unit according to the 10th embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the present invention will be described below withreference to the accompanying drawing.

First Embodiment

As shown in FIG. 1, in an OFDM system according to the first embodimentof the present invention, OFDM signals are transmitted for an OFDMsignal transmission apparatus 100 having a plurality of transmissionantennas 101 a and 101 b. The transmitted OFDM signals are received byan OFDM signal reception apparatus 200 having a plurality of receptionantennas 201 a and 201 b. In the system to be described here, the OFDMsignal transmission apparatus 100 has the two transmission antennas 101a and 101 b and the OFDM signal reception apparatus 200 has the tworeception antennas 201 a and 201 b. However, the present invention isnot limited to this and is also effective in a system having three ormore transmission antennas and reception antennas.

In this embodiment, as schematically shown in FIGS. 2A and 2B, the OFDMsignal transmission apparatus 100 forms two OFDM signals from twodifferent transmission data and transmits the OFDM signals from thedifferent transmission antennas 101 a and 101 b. In the first OFDMsignal shown in FIG. 2A, transmission data DATA_a (N, K) is superposed.In the second OFDM signal shown in FIG. 2B, transmission data DATA_b (N,K) is superposed. DATA_a (N, K) indicates data transmitted from thetransmission antenna 101 a and, more particularly, a signal transmittedby K subcarriers of the Nth symbol. DATA_b (N, K) indicates datatransmitted from the transmission antenna 101 b and, more particularly,a signal transmitted by K subcarriers of the Nth symbol. A pilotsubcarrier will be described later.

Let Haa be the transfer function of the channel (the transfer functionof the channel will be referred to as a channel response valuehereinafter) from the transmission antenna 101 a to the receptionantenna 201 a, Hab be the channel response value from the transmissionantenna 101 a to the reception antenna 201 b, Hba be the channelresponse value from the transmission antenna 101 b to the receptionantenna 201 a, and Hbb be the channel response value from thetransmission antenna 101 b to the reception antenna 201 b. A receptionsignal RXa of the reception antenna 201 a and a reception signal RXb ofthe reception antenna 201 b are expressed by

$\begin{matrix}{\left\lbrack \frac{RXa}{RXb} \right\rbrack = {\begin{bmatrix}H_{aa} & H_{ba} \\H_{ab} & H_{bb}\end{bmatrix}\left\lbrack \frac{TXa}{TXb} \right\rbrack}} & (1)\end{matrix}$where TXa and TXb are transmission signals from the transmissionantennas 101 a and 101 b, respectively. When the reception signals RXaand RXb are multiplied by the inverse matrix of the matrix formed by thechannel response values Haa, Hab, Hba, and Hbb, the transmission signalsTXa and TXb can be demodulated.

In the first embodiment, a pilot subcarrier to transmit a known signalto be used to compensate for the residual phase error of a frequencyoffset or clock offset is used independently of a data subcarrier totransmit data. More specifically, in the reception mode, the residualphase error is detected and compensated by using the known signaltransmitted by the pilot subcarrier.

A description will be made here for comparison. Jpn. Pat. Appln. KOKAIPublication No. 2003-304216, an OFDM signal shown in FIG. 30A istransmitted from the first transmission antenna, and an OFDM signalshown in FIG. 30B is transmitted from the second transmission antenna.That is, the pilot subcarrier indicated by a hatched portion istransmitted from only the first transmission antenna, as shown in FIG.30A. No pilot subcarrier is transmitted from the second transmissionantenna, as shown in FIG. 30B. At the frequency corresponding to thepilot subcarrier, a null signal indicated by a blank portion istransmitted. Since the pilot subcarriers are transmitted without anyinterference, the reception characteristic does not degrade due to adirectional beam. However, the total transmission power of the pilotsubcarriers becomes low.

According to the first embodiment, a satisfactory receptioncharacteristic can be obtained while sufficiently ensuring the totaltransmission power of pilot subcarriers by transmitting them from thetwo transmission antennas 101 a and 101 b.

The OFDM signal transmission apparatus 100 shown in FIG. 1 will bedescribed next with reference to FIG. 3. The OFDM signal transmissionapparatus 100 comprises an encoder 102, serial-parallel converter 103,modulators 104 a and 104 b, serial-parallel converters 105 a and 105 b,pilot subcarrier insertion unit 106, and inverse fast fourier transform(IFFT) units 107 a and 107 b.

Input transmission data is a wireless packet having a structure to bedescribed later. The transmission data is encoded by the encoder 102.The encoded data is subjected to serial-parallel conversion by theserial-parallel converter 103 and divided into first transmission datacorresponding to the transmission antenna 101 a and second transmissiondata corresponding to the transmission antenna 101 b. The first andsecond transmission data are subcarrier-modulated by the modulators 104a and 104 b, respectively. As the modulation method of the modulators104 a and 104 b, for example, binary phase shift keying (BPSK),quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation(QAM), or 64QAM is used, although the present invention is not limitedto these methods.

The modulated data output from the modulator 104 a is divided into aplurality of first data subcarriers by the serial-parallel converter 105a. The modulated data output from the modulator 104 b is divided into aplurality of second data subcarriers by the serial-parallel converter105 b.

The modulated data (to be referred to as first data subcarriers andsecond data subcarriers hereinafter) allocated to the first and seconddata subcarriers are input to the pilot subcarrier insertion unit 106.In the pilot subcarrier insertion unit 106, some of the subcarriers ofOFDM signals are allocated to pilot subcarriers to transmit pilotsignals, and the remaining subcarriers are allocated to data subcarriersto transmit data signals.

More specifically, the pilot subcarrier insertion unit 106 inserts pilotsignals (to be referred to as first pilot subcarriers hereinafter inthis specification) to be transmitted by at least one first pilotsubcarrier between the first data subcarriers. In addition, the pilotsubcarrier insertion unit 106 inserts pilot signals (to be referred toas second pilot subcarriers hereinafter in this specification) to betransmitted by at least one second pilot subcarrier between the seconddata subcarriers. A set of first data subcarriers and first pilotsubcarriers will be referred to as a first subcarrier signal. A set ofsecond data subcarriers and second pilot subcarriers will be referred toas a second subcarrier signal.

The first and second subcarrier signals output from the pilot subcarrierinsertion unit 106 are subjected to inverse fast Fourier transform bythe IFFT units 107 a and 107 b, respectively. As a result of inversefast Fourier transform, the first and second subcarrier signals aremultiplexed as they are converted from the signals on the frequency axisto signals on the time axis. Hence, a first OFDM signal a and secondOFDM signal b as shown in FIGS. 2A and 2B are generated. The OFDMsignals a and b are sent to the transmission antennas 101 a and 101 bthrough radio transmission units (not shown) and transmitted from thetransmission antennas 101 a and 101 b, respectively.

The pilot subcarrier insertion unit 106 will be described next withreference to FIG. 4.

In the pilot subcarrier insertion unit 106, the first data subcarriersfrom the serial-parallel converter 105 a and the second data subcarriersfrom the serial-parallel converter 105 b are directly output to the IFFTunits 107 a and 107 b. At this time, first pilot subcarriers areinserted between the first data subcarriers, and second pilotsubcarriers are inserted between the second data subcarriers.

In this embodiment, four first pilot subcarriers and four second pilotsubcarriers are present.

A sequence generator 110 generates a pseudorandom noise (PN) sequencelike an M sequence. The first pilot subcarriers are generated by causingmultiplying units 111 a to 111 d to obtain the products between a PNsequence PN(i) and polarity data Sa(j) of the first pilot subcarriers,which are stored in a ROM 121 a. The second pilot subcarriers aregenerated by causing multiplying units 112 a to 112 d to obtain theproducts between the PN sequence PN(i) and polarity data Sb(j) of thesecond pilot subcarriers, which are stored in a ROM 121 b. A basebandsignal Pa(i,j) of the first pilot subcarriers transmitted from thetransmission antenna 101 a is expressed by the product of PN(i) andSa(j), which is given byPb(i,j)=PN(i)×Sa(j)  (2)where i is the symbol number in the time domain, and j, is the pilotsubcarrier number in the frequency domain. Similarly, a baseband signalPb(i,j) of the pilot subcarriers from the transmission antenna 101 b isexpressed by the product of PN(i) and Sb(j), which is given byPb(i,j)=PN(i)×Sb(j)  (3)

In the first embodiment, the number of pilot subcarriers transmittedfrom each of the transmission antennas 101 a and 101 b is four (j=1 to4). The polarity data Sa(j) and Sb(j) (j=1, 2, 3, 4) of the first andsecond pilot subcarriers transmitted from the transmission antennas 101a and 101 b are set as follows.Sa(1)=1,Sa(2)=1,Sa(3)=1,Sa(4)=−1  (4)Sb(1)=1,Sb(2)=−1,Sb(3)=1,Sa(4)=1  (5)

That is, the polarity data Sa(j) to be multiplied by the PN sequence inthe first pilot subcarriers output from the transmission antenna 101 aare different from the polarity data Sb(j) to be multiplied by the PNsequence in the second pilot subcarriers output from the transmissionantenna 101 b. Accordingly, the polarity pattern of the first pilotsubcarriers is different from that of the second pilot subcarriers. Thepolarity pattern of the first pilot subcarriers indicates the pattern ofcombination of the polarities of the first pilot subcarriers. Thepolarity pattern of the second pilot subcarriers indicates the patternof combination of the polarities of the second pilot subcarriers. Theeffect obtained by using different polarity patterns for the first andsecond pilot subcarriers will be described later in detail. The polaritydata of each pilot subcarrier and the PN sequence are expressed by realnumbers.

Instead, polarity data represented by a complex number or a PN sequencerepresented by a complex number can also be used.

The OFDM signal reception apparatus 200 shown in FIG. 1 will bedescribed next with reference to FIG. 5. The OFDM signal receptionapparatus 200 comprises fast Fourier transform (FFT) units 202 a and 202b, interference suppression circuit 203, residual phase error detector204, phase compensation units 205 a and 205 b, serial-parallel converter206, and decoder 207.

The OFDM signal received by the reception antenna 201 a is input to theFFT unit 202 a through a wireless reception unit (not shown). The signalis divided into subcarrier signals by Fourier transform. The OFDM signalreceived by the reception antenna 201 b is also subjected to Fouriertransform by the FFT unit 202 b and divided into subcarrier signals.

As shown in FIG. 1, in the signal received by the reception antenna 201a, the OFDM signals transmitted from the transmission antennas 101 a and101 b are superposed. In the signal received by the reception antenna201 b as well, the OFDM signals transmitted from the transmissionantennas 101 a and 101 b are superposed. The interference suppressioncircuit 203 suppresses interference to demultiplex and receive the OFDMsignals from the transmission antennas 101 a and 101 b. The interferencesuppression system for this is a known technique. In this example, asystem which multiplies the received signal by the inverse matrix of thematrix formed by channel responses represented by equation (1) will bedescribed. The inverse matrix of the matrix formed by channel responsesrepresented by equation (1) can be rewritten to

$\begin{matrix}\begin{matrix}{\begin{bmatrix}H_{aa} & H_{ba} \\H_{ab} & H_{bb}\end{bmatrix}^{- 1} = {\frac{1}{{{H_{aa}H_{bb}} - {H_{ba}H_{ab}}}}\begin{bmatrix}H_{bb} & {- H_{ba}} \\{- H_{ab}} & H_{aa}\end{bmatrix}}} \\{= \begin{bmatrix}A & B \\C & D\end{bmatrix}}\end{matrix} & (6)\end{matrix}$

When the inverse matrix represented by equation (6) is multiplied byreceived signal vectors generated from the received signals output fromthe reception antennas 201 a and 201 b, the OFDM signals from thetransmission antennas 101 a and 101 b are demultiplexed. In a multipathenvironment, the channel response value changes between the subcarriers.As a result, derivation of the coefficient of the inverse matrix andmultiplication of the inverse matrix are executed for each subcarrier.The signals demultiplexed by the interference suppression circuit 203are sent to the residual phase error detector 204.

The residual phase error detector 204 detects a residual component suchas a frequency offset or clock offset compensated by using the preambleof a wireless packet (not shown). The residual phase error detector 204also detects the residual phase error of the received signals RXa andRXb by using the known signal transmitted by the subcarrier and sendsthe residual phase error to the phase compensation units 205 a and 205b.

FIG. 6 shows the detection principle of the residual phase errordetector 204. An example will be described here in which residual phaseerror detection is applied to a signal without interference suppression.For a pilot subcarrier, the weight matrix (right-hand component ofequation (6)) represented by equation (6) is expressed by a unit matrixor a matrix obtained by weighting and combining the outputs from the FFTunits 202 a and 202 b to maximize the S/N ratio, i.e., a maximum ratiocombining (MRC) matrix. When pilot subcarriers of two values generatedfrom PN sequences of two values are transmitted by using the twotransmission antennas 101 a and 101 b, 2²=4 candidate received signalpoints (1,1), (1,−1), (−1,1), and (−1,−1) are present in the OFDM signalreception apparatus before interference suppression, as shown in FIG. 6.For example, (1,−1) represents that a modulated signal “1” istransmitted at a subcarrier from the transmission antenna 101 a, and amodulated signal “−1” is transmitted at a subcarrier from thetransmission antenna 101 b.

As in the first embodiment, when the first and second pilot subcarriersgenerated from the common PN sequence are transmitted from thetransmission antennas 101 a and 101 b, the combination of receptionsignal points is (1,1) and (−1,−1), or (1,−1) and (−1,1). Thiscombination does not change during wireless packet reception. Forexample, when the combination of reception signal points is (1,−1) and(−1,1), it looks for the OFDM signal reception apparatus as if a BPSKsignal were transmitted from a single transmission antenna.

A case in which a residual phase error is detected by using pilotsubcarriers Pa(1) and Pb(1) transmitted by the (−k+1)th subcarrier willbe described next. Only the FFT unit 202 a connected to the receptionantenna 201 a is taken into consideration. Let Haa be the channelresponse value from the transmission antenna 101 a to the receptionantenna 201 a in the (−k+1)th subcarrier. Let Hba be the channelresponse value from the transmission antenna 101 b to the receptionantenna 201 a. When the polarities of the pilot subcarriers arerepresented by equations (4) and (5), polarities corresponding to the(−k+1)th pilot subcarrier are Sa(1)=1 and Sb(1)=1. Since the pilotsignal in which the signals from the two transmission antennas aremultiplexed is multiplied by the channel response value Haa+Hba, the twopoints (1,1) and (−1,−1) are received. Hence, the residual phase errordetector calculates the channel response value Haa+Hba by using thechannel response values Haa and Hba and creates the reference signalpoints (1,1) and (−1,−1).

Assume that (1,1) is transmitted by the next OFDM symbol, and thereceived signal point at this time is “next symbol” in FIG. 6. At thistime, the residual phase error detector 204 can detect, as the residualphase error, a phase difference θ between the next symbol and thecurrent reception signal point (1,1). The residual phase error value canbe obtained from both of the plurality of outputs, i.e., the output fromthe system of the reception antenna 201 a and the output from the systemof the reception antenna 201 b. In this case, the average value orweighed average value of the two values can be output to the phasecompensators 205 a and 205 b.

In the first embodiment, residual phase error detection using pilotsubcarriers is executed without using interference suppression. Instead,the residual phase error may be detected after interference suppressionis executed. In this case, the received signal points of pilotsubcarriers appear equal in number to the transmission signal pointsfrom the transmission antennas 101 a and 101 b. When residual phaseerror detection using pilot subcarriers is executed after interferencesuppression, the S/N ratio of the pilot subcarriers is low. Hence, theestimation accuracy degrades.

The phase compensation units 205 a and 205 b execute phase rotation forthe reception signals in correspondence with the residual phase error,thereby compensating for the phase. The two reception signals afterphase compensation are converted into a serial signal by theserial-parallel converter 206 and decoded by the decoder 207 so that areception signal corresponding to the transmission signal is obtained.

As described above, the pilot subcarriers are used to detect theresidual phase error. If the signal to noise ratio of the pilotsubcarriers are low, the performance of the residual phase errordetection will degrade. In this case, since the phase compensation units205 a and 205 b execute phase compensation on the basis of the wrongresidual phase error detection result, all data subcarriers areerroneously received at a high probability. Hence, it's no exaggerationto say that the reception power of pilot subcarriers determines thereception performance of the OFDM signal reception apparatus. To solvethis problem, in this embodiment, the first pilot subcarrier transmittedfrom the transmission antenna 101 a and the second pilot subcarriertransmitted from the transmission antenna 101 b have differentpolarities, as described above.

FIG. 7 schematically shows the beam patterns of each transmissionantenna 101 a and 101 b and the beam patterns combined with the antennas101 a and 101 b when pilot subcarriers having the same polarity patternare transmitted from the antennas 101 a and 101 b. The polarity dataSa(1), Sa(2), Sa(3), and Sa(4) of the first pilot subcarrierstransmitted from the transmission antenna 101 a are represented byequation (4). The polarity data Sb(1), Sb(2), Sb(3), and Sb(4) of thesecond pilot subcarriers transmitted from the transmission antenna 101 bare given bySb(1)=1,Sb(2)=1,Sb(3)=1,Sb(4)=−1  (7)

The transmission antennas 101 a and 101 b are assumed to beomni-directional antennas as shown on the upper side of FIG. 7. Whenpilot subcarriers of the same polarity are transmitted simultaneouslyfrom the antennas 101 a and 101 b, the transmission signals interferewith each other, and their combined beam patterns form directionalbeams. On the basis of the IEEE 802.11a standard, the interval (4.4 MHz)between the pilot subcarriers is much smaller than the center frequency(carrier frequency) of 5.5 GHz of the OFDM signal. Hence, thedirectional beams of the four pilot subcarriers are directed in almostthe same direction, as shown on the lower side of FIG. 7. As a result,there will be an area where the received powers of all the four pilotsubcarriers largely decrease, i.e., a dead zone where the performance ofthe OFDM signal reception apparatus degrades.

FIG. 8 schematically shows the beam patterns of each transmissionantenna 101 a and 101 b and the beam patterns combined with antennas 101a and 101 b when the polarity pattern of the first pilot subcarrierstransmitted from the antenna 101 a and that of the second pilotsubcarriers transmitted from the antenna 101 b are different. Thepolarity data Sa(1), Sa(2), Sa(3), and Sa(4) of the first pilotsubcarriers transmitted from the transmission antenna 101 a arerepresented by equation (4). The polarity data Sb(1), Sb(2), Sb(3), andSb(4) of the second pilot subcarriers transmitted from the transmissionantenna 101 b are represented by equation (5).

In accordance with equations (4) and (5), for example, the phasedifference between the first pilot subcarrier controlled in accordancewith the polarity data Sa(1) and the second pilot subcarrier controlledin accordance with the polarity data Sb(1) is 0. To the contrary, thephase difference between the first pilot subcarrier controlled inaccordance with the polarity data Sa(2) and the second pilot subcarriercontrolled in accordance with the polarity data Sb(2) is 180°. As aresult, as shown in, e.g., the lower side of FIG. 8, the directionalbeam formed by the pilot subcarriers having the polarities Sa(1) andSb(1) and that formed by the pilot subcarriers having the polaritiesSa(2) and Sb(2) are directed in directions different by 180°.

FIG. 9 shows the average normalized reception level of pilot subcarriersin the receiver when the pilot subcarriers are transmitted by using theembodiment of the present invention. Two transmission antennas are used.Four pilot subcarriers Pa(1), Pa(2), Pa(3), and Pa(4) and four pilotsubcarriers Pb(1), Pb(2), Pb(3), and Pb(4) using equations (4) and (5)are used. As for the polarity of each pilot subcarrier, the centerfrequency of the OFDM signal is 5 GHz, the signal bandwidth is about 20MHz, the antenna element interval is a half wavelength, and each elementis omni-directional. As the channel model, “Channel model D (NLOS)”shown in IEEE 802.11-03-940/rl “TGn Channel model” is used. The X-axisin FIG. 9 represents the angle viewed from the transmission antenna, andthe Y-axis represents the received power of four pilot subcarrierscorresponding to a certain transmission angle.

As is apparent from FIG. 9, at a certain angle, the power of a pilotsubcarrier decreases as compared to other angles. At that point,however, the reception powers of the remaining pilot subcarriers arehigh. For this reason, the receiver can compensate for the residualphase error by using the pilot subcarriers of high level.

FIG. 10 shows the reception power when the pilot subcarriers aretransmitted by using equations (4) and (7). That is, FIG. 10 shows thecharacteristic when pilot signals having the same polarity are used. Asis apparent from FIG. 10, at a certain angle, the power of a pilotsubcarrier decreases as compared to other angles. The remaining pilotsubcarriers also have this tendency. Hence, the powers of all the pilotsubcarriers decrease simultaneously. For this reason, it is difficultfor a receiver depending on the angle to compensate for the residualphase error by using the pilot subcarriers.

On the receiving side, even when the reception power of one pilotsubcarrier is low, the reception powers of the remaining pilotsubcarriers are high at a high probability by using the firstembodiment. Since the dead zone where the reception powers of all thepilot subcarriers decrease simultaneously can be reduced, the area wherehigh-quality reception is possible widens.

In the first embodiment, it is not necessary to especially increase thetransmission powers of subcarriers for all the transmission antennas 101a and 101 b since the pilot subcarriers are transmitted from allantennas.

For this reason, the composite triple beat does not increase, and theinput dynamic range of the D/A converter need not particularly bewidened.

In the above description, the OFDM signal transmission apparatus 100 hasthe two transmission antennas 101 a and 101 b. However, the presentinvention can also be extended to an OFDM apparatus having three or moretransmission antennas. FIGS. 11A and 11B show examples of two kinds ofpolarity patterns of the first to fourth pilot subcarriers which aretransmitted from each of four transmission antennas 101 a to 101 d.FIGS. 11C and 11D show examples of two kinds of polarity patterns of thefirst to fourth pilot subcarriers which are transmitted from each ofthree transmission antennas 101 a to 101 c. Two kinds of pilotsubcarrier polarities 1 and 2 are prepared for each of the first tofourth pilot subcarriers. Pilot subcarrier polarity 1 is a polaritypattern when a real number is used. Pilot subcarrier polarity 2 is apolarity pattern when an imaginary number is used. Pilot subcarrierpolarity 2 might be generated by using the coefficient of a Fouriermatrix.

In pilot subcarrier polarity 1 for the four transmission antennas shownin FIG. 11A, the polarities of the first pilot subcarriers are Sa(1)=1,Sa(2)=1, Sa(3)=1, and Sa(4)=−1. The polarities of the second pilotsubcarriers are Sb(1)=1, Sb(2)=−1, Sb(3)=1, and Sb(4)=1. The polaritiesof the third pilot subcarriers are Sc(1)=1, Sc(2)=−1, Sc(3)=−1, andSc(4)=−1. The polarities of the fourth pilot subcarriers are Sd(1)=1,Sd(2)=1, Sd(3)=−1, and Sd(4)=1. For the pilot subcarriers transmittedfrom a certain frequency, consider vectors each having the respectivepolarities as elements. Four pilot subcarriers are transmitted from eachtransmission antenna. Since there are four transmission antennas, thefour following vectors each having four elements as the polarity datacan be defined as the polarity pattern.s(1)=[1,1,1,1]s(2)=[1,−1,−1,1]s(3)=[1,1,−1,−1]s(4)=[−1,1,−1,1]  (8)

In this case, s(1) to s(4) are vectors different from each other. Forexample, the vector s(1) does not change to another vector even when itis multiplied by scalar value. When the vector of a pilot subcarriertransmitted from a certain frequency is different from that of a pilotsubcarrier transmitted from another frequency, the directional beams ofthe respective pilot subcarriers are directed in different directions.Hence, the dead zone can be reduced. Note that s(1) to s(4) areorthogonal to each other. Even when they are not orthogonal to eachother, the directional beams can be directed in different directions.

In pilot subcarrier polarity 2 for the four transmission antennas shownin FIG. 11B, the polarities of the first pilot subcarriers are Sa(1)=1,Sa(2)=1, Sa(3)=1, and Sa(4)=−1. The polarities of the second pilotsubcarriers are Sb(1)=1, Sb(2)=−j, Sb(3)=−1, and Sb(4)=−j. Thepolarities of the third pilot subcarriers are Sc(1)=1, Sc(2)=−1,Sc(3)=1, and Sc(4)=1. The polarities of the fourth pilot subcarriers areSd(1)=1, Sd(2)=j, Sd(3)=−1, and Sd(4)=j, where j is the imaginary unit.In this case as well, for the pilot subcarriers transmitted from acertain frequency, consider vectors each having the respectivepolarities as elements, as described above. The vectors are differentfrom each other in the complex domain. They are orthogonal to each otherin the complex domain. However, they need not always be orthogonal toeach other.

In pilot subcarrier polarity 1 for the three transmission antennas shownin FIG. 11C, the polarities of the first pilot subcarriers are Sa(1)=1,Sa(2)=1, Sa(3)=1, and Sa(4)=−1. The polarities of the second pilotsubcarriers are Sb(1)=1, Sb(2)=−1, Sb(3)=1, and Sb(4)=−1. The polaritiesof the third pilot subcarriers are Sc(1)=1, Sc(2)=−1, Sc(3)=−1, andSc(4)=−1. In this case as well, for the pilot subcarriers transmittedfrom a certain frequency, consider vectors each having the respectivepolarities as elements, as described above. The vectors are differentfrom each other.

In pilot subcarrier polarity 2 for the three transmission antennas shownin FIG. 11D, the polarities of the first pilot subcarriers are Sa(1)=1,Sa(2)=1, Sa(3)=1, and Sa(4)=−1. The polarities of the second pilotsubcarriers are Sb(1)=1, Sb(2)=−j, Sb(3)=−1, and Sb(4)=−j. Thepolarities of the third pilot subcarriers are Sc(1)=1, Sc(2)=−1,Sc(3)=1, and Sc(4)=1. In this case as well, for the pilot subcarrierstransmitted from a certain frequency, consider vectors each having therespective polarities as elements, as described above. The vectors aredifferent from each other. Each vector does not change to another vectoreven when it is multiplied in the complex domain.

When the locations of pilot subcarriers are determined in this way, thedirectional beams of the respective pilot subcarriers are directed indifferent directions. For this reason, the dead zone can be reduced.

In the complex domain, the pilot subcarrier polarities shown in FIGS.11A to 11D can be given by

$\begin{matrix}{{s_{k}(i)} = {\exp\left( {\frac{j\pi}{2}\left( {k - 1} \right){\mathbb{i}}} \right)}} & (9)\end{matrix}$where s_(k)(i) is the polarity of the pilot subcarrier, j is theimaginary unit, i is the number of the pilot subcarrier, and k is theantenna number of the transmission antenna. For example, the firstelement of k represents the signal transmitted from the transmissionantenna 101 a, and the second element of k represents the signaltransmitted from the transmission antenna 101 b.

According to equation (9), the phase difference between the first tofourth pilot subcarriers transmitted from the antenna 101 b of k=2 is−90°. The phase difference between the pilot subcarriers transmittedfrom the antenna 101 c of k=3 is −180°. The phase difference between thepilot subcarriers transmitted from the antenna 101 d of k=4 is −270°.When the transmission antenna changes, the phase difference betweenpilot subcarriers of the antenna changes. Hence, as described above,since the directional beams corresponding to the respective pilotsubcarriers are directed in different directions, the dead zone can bereduced.

The phase difference of −90° equals the phase difference of 270° and thephase difference of −270° equals the phase difference of 90°. To expressFIGS. 11A to 11D faithfully, a minus sign is necessary in theexponential term of equation (9). However, since the same result can beobtained without any minus sign as described above, the sign is omittedin equation (9).

According to FIGS. 11A and 11B, of the pilot subcarriers transmittedfrom the first transmission antenna 101 a, the fourth pilot subcarrierhas an inverted polarity, unlike the first to third pilot subcarriers,as represented by[S _(k)(1),S _(k)(2),S _(k)(3),S _(k)(4)]=[s _(k)(1),s _(k)(2),s_(k)(3),−s _(k)(4)]  (10)

In other words, the polarities of the first to fourth pilot subcarriersare given by

$\begin{matrix}{{s_{k}(i)} = \left\{ \begin{matrix}{\exp\left( {\frac{j\pi}{2}\left( {k - 1} \right)i} \right)} & {{i = 0},1,2} \\{- {\exp\left( {\frac{j\pi}{2}\left( {k - 1} \right)i} \right)}} & {i = 3}\end{matrix} \right.} & (11)\end{matrix}$

More generally, depending on the pilot subcarrier number i, the polarityof a pilot subcarrier is given by

$\begin{matrix}{{{s_{k}(i)} = {\exp\left( {\frac{j\pi}{2}\left( {k - 1} \right)i} \right)}}{or}} & (12) \\{{s_{k}(i)} = {- {\exp\left( {\frac{j\pi}{2}\left( {k - 1} \right)i} \right)}}} & (13)\end{matrix}$

In the above description, i is the number of a pilot subcarrier.However, i may be changed to the frequency of the pilot subcarrier. Morespecifically, for example, −21, −7, +7, and 21 are used as the value i.In consideration of the periodicity of the Fourier transform function,the pilot subcarrier polarity can also be expressed by

$\begin{matrix}{{s_{k}(i)} = {\exp\left( \frac{{{j2\pi}\left( {k - 1} \right)}{\mathbb{i}}}{N} \right)}} & (14)\end{matrix}$where s_(k)(i) is the polarity of the pilot subcarrier, j is theimaginary unit, i is the frequency of the pilot subcarrier, k is theantenna number of the transmission antenna, and N is the number of inputpoints in inverse Fourier transform.

Even in equation (14), when the transmission antenna changes, the phasedifference between the pilot subcarriers changes. Hence, the dead zonecan be reduced. In consideration of a Fourier transform pair, theexpression of equation (14) is equivalent to cyclic shifting thetransmission signal sample by sample for each transmission antenna alongthe time domains.

Other embodiments of the present invention will be described next. Theother embodiments to be described below are different from the firstembodiment in the pilot subcarrier insertion unit 106 in the OFDM signaltransmission apparatus 100.

Second Embodiment

A pilot subcarrier insertion unit 106 according to the second embodimenthas ROMs 121 a and 121 b which store polarity data for first and secondpilot subcarriers, and a subcarrier pattern controller 122, as shown inFIG. 12. As shown in FIG. 13, the ROMs 121 a and 121 b store threepatterns (pattern A, pattern B, and pattern C) of polarity datarepresenting polarities Sa(1) to Sa(4) of the first pilot subcarriersand polarity data representing polarities Sb(1) to Sb(4) of the secondpilot subcarriers. Which polarity data of the pattern A, pattern B, andpattern C is to be read out from the ROMs 121 a and 121 b is determinedby address data given to the ROMs 121 a and 121 b.

In the second embodiment, the polarities of the first and second pilotsubcarriers transmitted from transmission antennas 101 a and 101 b arenot fixed but change for each wireless packet. More specifically,polarity data of different patterns are read out from the ROMs 121 a and121 b for each wireless packet and multiplied, by multipliers 111 a to111 d and 112 a to 112 d, by a PN sequence generated by a PN sequencegenerator 110.

A wireless packet counter 123 arranged outside the pilot subcarrierinsertion unit 106 shown in FIG. 12 counts the number of wirelesspackets in transmission data input to an encoder 102 shown in FIG. 3 andtransfers the count value to the subcarrier pattern controller 122.Every time the count value of the wireless packet counter 123 isincremented by one, the subcarrier pattern controller 122 changesaddress data to be given to the ROMs 121 a and 121 b. The subcarrierpattern controller 122 thus changes the patterns of the polarity data ofthe first and second pilot subcarriers, which are read out from the ROMs121 a and 121 b.

For example, when a wireless packet is transmitted, the polarity data ofthe pattern A is read out. When the next wireless packet is transmitted,the polarity data of the pattern B is read out. When the third wirelesspacket is transmitted, the polarity data of the pattern C is read out.As a result, the polarity pattern of pilot subcarriers is changed foreach wireless packet. Polarity data pattern change by the subcarrierpattern controller 122 is done, e.g., at random for each wirelesspacket.

The polarity data read out from the ROMs 121 a and 121 b are input tothe multipliers 111 a to 111 d and multiplied by the PN sequencegenerated by the PN sequence generator 110 so that first and secondpilot subcarriers are generated, as in the first embodiment. Thegenerated first and second pilot subcarriers are inserted between firstdata subcarriers and between second data subcarriers, respectively, sothat first and second subcarriers are generated.

When the first and second subcarriers are input to IFFT units 107 a and107 b shown in FIG. 3, first and second OFDM signals are generated. Thefirst and second OFDM signals are sent to the transmission antennas 101a and 101 b through radio transmission units (not shown), as shown inFIG. 3, and transmitted from the transmission antennas 101 a and 101 b.

According to the second embodiment, the pattern of directional beamsformed by the antennas 101 a and 101 b changes between transmission ofpilot subcarriers controlled by, e.g., the polarity data of the patternA and transmission of pilot subcarriers controlled by, e.g., thepolarity data of the pattern B.

In an OFDM signal reception apparatus which is placed in a zone wherethe received power is low for the pilot subcarriers of, e.g., thepattern A, the probability that the received power of pilot subcarriersrecovers is high when pilot subcarriers of the pattern B different fromthe pattern A are transmitted. Hence, when the pilot subcarrier patternis changed for each wireless packet, the dead zone can be reduced.

The pattern of polarity data of pilot subcarriers need not always bechanged at random. For example, of various patterns of polarity data ofpilot subcarriers, a pattern which ensures a satisfactory receptionperformance may be stored in correspondence with each OFDM signalreception apparatus. In this case, pilot subcarriers can be transmittedby using the pattern stored in correspondence with the OFDM signalreception apparatus at the transmission destination.

Third Embodiment

The third embodiment of the present invention will be described next. Inthe third embodiment, the polarity pattern of pilot subcarriers ischanged only when an error has occurred in a preceding transmittedwireless packet, and the packet is for retransmission. As shown in FIG.14, a pilot subcarrier insertion unit 106 according to the thirdembodiment is the same as the pilot subcarrier insertion unit 106 shownin FIG. 12 except that the wireless packet counter 123 in FIG. 12 isreplaced with a retransmission detector 124.

As shown in FIG. 15, a wireless packet contains a unique word to be usedfor synchronization by an OFDM signal reception apparatus, a sourceaddress field to specify the transmission source address(the address ofOFDM signal transmission apparatus), a destination address field tospecify the transmission destination address(the address of OFDM signalreception apparatus), a retransmission field which indicates whether thewireless packet is a retransmission packet, and an error check field todetermine whether an error has occurred in each field. A plurality ofdata symbols follow.

A transmission signal input to an encoder 102 shown in FIG. 3 is alsoinput to the retransmission detector 124. The retransmission detector124 analyzes the retransmission field in the transmission signal, i.e.,wireless packet. If the wireless packet is a retransmission packet, theretransmission detector 124 notifies a subcarrier pattern controller 122of it. Upon being notified that the wireless packet is a retransmissionpacket, the subcarrier pattern controller 122 changes address data to begiven to ROMs 121 a and 121 b, thereby controlling the patterns ofpolarity data of first and second pilot subcarriers, which are read outfrom the ROMs 121 a and 121 b. As a result, a wireless packet containingpilot subcarriers of a polarity pattern different from that of pilotsubcarriers of the preceding transmitted wireless packet is transmittedto the same transmission partner.

In the third embodiment, whether the wireless packet is a retransmissionpacket is determined by analyzing the retransmission field. Instead, anupper-level layer (medium access control (MAC) layer in the IEEE 802.11astandard) which executes wireless access control may directly notify thesubcarrier pattern controller that the wireless packet is for aretransmission.

As described above, in the third embodiment, the retransmission packetis transmitted to the same transmission partner by using a polaritypattern different from that of the pilot subcarriers of the wirelesspacket preceding transmitted from an OFDM signal transmission apparatus100. As a result, the pattern of directional beams formed by theplurality of transmission antennas changes in retransmission. Hence, theOFDM signal reception apparatus can correctly receive the retransmittedpacket at a high probability.

Fourth Embodiment

A pilot subcarrier insertion unit 106 according the fourth embodiment ofthe present invention includes two PN sequence generators 110 a and 110b, as shown in FIG. 16. The first PN sequence generator 110 a generatesa first PN sequence PNa which modulates first pilot subcarrierstransmitted from a transmission antenna 101 a. The second PN sequencegenerator 110 b generates a second PN sequence PNb which modulatessecond pilot subcarriers transmitted from a transmission antenna 101 b.

The polarity patterns of the first and second subcarriers may be eitherthe same or different. A case in which the same polarity data S are usedfor the first and the second subcarriers, will be described here. Thefirst pilot subcarriers are modulated as follows in accordance with thePN sequence PNa and pilot subcarrier polarity data S.Pa(i,j)=PNa(i)×S(j)  (15)

The second pilot subcarriers are modulated as follows in accordance withthe PN sequence PNb and pilot subcarrier polarity data S.Pb(i,j)=PNb(i)×S(j)  (16)

FIGS. 17A and 17B show first and second OFDM signals containing datasubcarriers and pilot subcarriers modulated in this way. The first andsecond OFDM signals are sent to IFFT units 107 a and 107 b andtransmitted from the transmission antennas 101 a and 101 b,respectively.

FIG. 18 shows the detection principle of a residual phase error detector204 shown in FIG. 5 according to the fourth embodiment. A case in whichthe residual phase error is estimated before interference suppressionwill be described. When PN sequences of two values and pilot subcarriersof two values are transmitted from the two transmission antennas 101 aand 101 b, as in FIG. 6 of the first embodiment, 2²=4 candidatereception signal points (1,1), (1,−1), (−1,1), and (−1,−1) are present.In the first embodiment, a combination of reception signal points (1,1)and (−1,−1), or (1,−1) and (−1,1) are present. In the fourth embodiment,all the four reception signal points can appear for each OFDM symbolbecause the pilot subcarriers transmitted from the transmission antennas101 a and 101 b are modulated by different PN sequences.

In the fourth embodiment, for example, a case in which the combinationof (1,−1) and (−1,1) is transmitted and a case in which the combinationof (1,1) and (−1,−1) is transmitted can be considered. In the formercombination, the phase difference between the signals from thetransmission antennas 101 a and 101 b is 180°. In the lattercombination, the phase difference is 0°. Since the directional beam oftransmission changes between the transmission of the former combinationand the transmission of the latter combination, the reception powerchanges. As described in the first embodiment, the four candidatereception signal points (1,−1), (−1,1), (1,1), and (−1,−1) can beobtained by combining the channel response from the transmissionantennas to the reception antennas with the signals transmitted by thepilot subcarriers.

The residual phase error measuring method will be described next. Assumethat the received symbol of a pilot subcarrier is (1,1). In this case,in the first embodiment, (1,1) is transmitted again as the next symbol,or (−1,−1) is transmitted. It looks on the receiving side as if a BPSKsignal were received from a single antenna. Hence, the reception powerdoes not change.

In the fourth embodiment, (−1,1) can also be transmitted as the nextsymbol. Hence, the received symbol containing a phase error can be “nextsymbol 1” or “next symbol 2” shown in FIG. 18. When the received signalpoint of the next symbol is (1,1) or (−1,−1), the residual phase errordetector 204 detects a phase difference θ1 as the residual phase error.On the other hand, when the reception signal point of the next symbol is(−1,1) or (1,−1), the residual phase error detector 204 creates thereplica of the reception signal (−1,1) from the current channel responsevalue and detects a phase difference θ2 between (−1,1) and “next symbol2” as the residual phase error.

Phase compensation units 205 a and 205 b multiplex the phase rotationfor the reception signals in correspondence with the residual phaseerror, thereby compensating for the phase. The two received signalsafter phase compensation are converted into a serial signal by aserial-parallel converter 206 and decoded by a decoder 207 so that areception signal corresponding to the transmission signal is obtained.In the fourth embodiment, the residual phase error is measured beforeinterference suppression. The residual phase error can also be detectedafter interference suppression. In this case, a pilot subcarriertransmitted from a single antenna appears as the output afterinterference suppression. Hence, only two reception signal pointsappear.

As described above, according to the fourth embodiment, as shown in FIG.17, the combination of (1,1) and (−1,−1) or combination of (1,−1) and(−1,1) is transmitted in each OFDM symbol. Hence, the reception powerchanges for each OFDM symbol. Even when the reception power of pilotsubcarriers becomes low in a certain OFDM symbol, and the residual phaseerror cannot be detected correctly, the received power can recover inthe next symbol and the phase error can be detected correctly. As aconsequence, the dead zone where the received powers of all pilotsubcarriers decrease can be eliminated.

Fifth Embodiment

As shown in FIG. 19, a pilot subcarrier insertion unit 106 according tothe fifth embodiment of the present invention is formed by combining thesecond embodiment shown in FIG. 12 or the third embodiment shown in FIG.14 and the fourth embodiment shown in FIG. 16. More specifically, in thefifth embodiment, the pilot subcarrier polarity changes betweentransmission antennas, and the PN sequence to modulate the pilotsubcarrier changes between the transmission antennas. Hence, thebaseband signals of pilot subcarriers corresponding to transmissionantennas 101 a and 101 b are respectively given byPa(i,j)=PNa(i)×Sa(j)  (17)Pb(i,j)=PNb(i)×Sb(j)  (18)

In this case, pilot subcarriers shown in FIGS. 20A and 20B are obtained.Directional beams formed by the transmission antennas 101 a and 101 b incorrespondence with the pilot subcarriers are directed in differentdirections for each frequency and also in different directions along thesymbol in the time domain or OFDM symbols. Hence, even when thereception level of pilot subcarriers is low at a certain frequency ortime, the pilot subcarriers can be received at another frequency or OFDMsymbol so that the dead zone can be reduced.

Sixth Embodiment

In a pilot subcarrier insertion unit 106 according to the sixthembodiment of the present invention, as shown in FIG. 21, PN sequencesfrom PN sequence generators 110 a and 110 b are input to transmissiondiversity circuits 125 a and 125 b so that pilot subcarriers aretransmitted asPa(2i−1,j)=PN(2i−1)  (19)Pa(2i,j)=−PN(2i)*  (20)Pb(2i−1,j)=PN(2i)  (21)Pb(2i,j)=−PN(2i−1)  (22)where * is the complex conjugate. As indicated by equations (19) to(22), the PN sequences are transmitted by using transmission diversityusing two transmission antennas 101 a and 101 b and two symbols. Thetransmission diversity method represented by equations (19) to (22) isthe same as that disclosed in U.S. Pat. No. 6,185,258B1.

The detailed pilot subcarrier signals given by equations (19) to (22)are applied to the (−k+1)th subcarrier and (k−4)th subcarrier shown inFIGS. 22A and 22B. Equations (19) to (22) indicate the jth pilotsubcarrier. For another subcarrier, the transmission method can bechanged. More specifically, this can be expressed byPa(2i−1,j)=PN(2i  (23)Pa(2i,j)=−PN(2i−1)  (24)Pb(2i−1,j)=PN(2i−1)  (25)Pb(2i,j)=−PN(2i)*  (26)

The detailed pilot subcarriers given by equations (23) to (26) areapplied to the (−k+4)th subcarrier and (k−1)th subcarrier shown in FIGS.22A and 22B. In a residual phase error detector 204 in an OFDM signalreception apparatus shown in FIG. 5, for example, when decodingcorresponding to transmission diversity is executed by using a decodingmethod disclosed in U.S. Pat. No. 6,185,258B1, the Signal to Noise ratioof the pilot subcarriers can be maximized.

As is apparent from equations (19) to (26), in this embodiment,transmission diversity is executed by using a two-OFDM symbol duration.No diversity gain is obtained when only one symbol is received. FIGS.23A and 23B show signals obtained by modulating the (−k+4)th subcarrierby using equations (23) to (26). As shown in FIGS. 23A and 23B, insymbol 1, the residual phase error is detected by the received signal ofonly one symbol. From the second symbol, since the preceding receivedsymbol can be used, the residual phase error of symbol 2 can be detectedby using the received signals of symbols 1 and 2. Similarly, theresidual phase error of symbol 3 can be detected by using the receivedsignals of symbols 2 and 3. That is, the phase error can be detected bymaking two consecutive symbols. All residual phase errors can also bedetected by using only a single symbol which is currently received.

As described above, according to the sixth embodiment, pilot subcarriersare transmitted by using transmission diversity. Hence, the residualphase error can accurately be detected, and the performance at thereceiver can be improved.

Seventh Embodiment

A pilot subcarrier insertion unit 106 according to the seventhembodiment of the present invention includes subcarrier locating devices126 a and 126 b to make the locations of pilot subcarriers and datasubcarriers change between transmission antennas 101 a and 101 b, asshown in FIG. 24. The subcarrier locating devices 126 a and 126 b makethe locations of pilot subcarriers and data subcarriers change betweenthe transmission antennas. The pilot subcarrier insertion unit 106 willbe described next in detail with reference to FIG. 24.

Modulated signals for the transmission antenna 101 a, which are obtainedby multiplying a PN sequence by polarities Sa(1) to Sa(4), are input tothe subcarrier locating device 126 a as pilot subcarriers Pa(1) toPa(4). The subcarrier locating device 126 a rearranges the datasubcarriers and pilot subcarriers and inputs them to an IFFT unit 107 a.The processing of signals for the transmission antenna 101 b is also thesame, and a description thereof will be omitted.

FIGS. 25A and 25B show the subcarrier location in the seventhembodiment. As shown in FIGS. 25A and 25B, of the subcarriers, the pilotsubcarriers are transmitted from only a single transmission antenna.Pilot subcarriers PN(1) modulated by the PN sequence are transmittedfrom the transmission antenna 101 a as the −kth subcarriers. Datasubcarriers (DATA) are transmitted from the transmission antenna 101 b.For the (−k+2)th subcarrier as well, pilot subcarriers are transmittedfrom the transmission antenna 101 a, and the data subcarriers (DATA) aretransmitted from the transmission antenna 101 b.

Since data signals can be assumed to be random, the correlation betweenthe first data subcarriers transmitted from the transmission antenna 101a and the second data subcarriers transmitted from the transmissionantenna 101 b is generally low. For this reason, the phase differencebetween the subcarriers transmitted from the transmission antenna 101 aand those transmitted from the transmission antenna 101 b changesbetween the −kth subcarrier and the (−k+2)th subcarrier. Hence, thedirectional beam of the pilot subcarrier transmitted by the kthsubcarrier is different from the directional beam of the pilotsubcarrier transmitted by the (−k+2)th subcarrier at a high probability.

According to the seventh embodiment, since the probability that thereception powers of all the pilot subcarriers decrease simultaneouslydue to the influence of null points is very low, no dead zone is formed.In addition, even when the power of pilot subcarriers happens to be lowin one symbol interval, the data signal changes at a high probabilitybetween the current symbol and the next symbol. Hence, the receptionpower of the pilot subcarriers can recover in the next symbol at a highprobability. As described above, according to the seventh embodiment,the pilot subcarrier reception probability can be increased, and thedead zone can be reduced.

The pilot subcarrier location can also be changed in wireless packets.In, e.g., the IEEE 802.11a standard, a pilot signal to estimate thechannel response of all subcarriers is inserted in the unique word shownin FIG. 15. On the basis of the pilot signal, the channel responserepresented by equation (1) can be obtained. In the data symbol, thepilot signal is transmitted by using only the pilot subcarrier. For thisreason, if the temporal variation in the channel is fast, tracking ofthe channel response is difficult. However, when the pilot subcarrierlocation is changed between wireless packets, and the pilot signal isalso transmitted from other subcarriers, tracking of the channelresponses of other subcarriers can be executed. Hence, when this systemis used, accurate reception is possible. In the seventh embodiment, thepolarities of pilot subcarriers are the same. However, the same settingas in the first to sixth embodiment can also be employed.

Eighth Embodiment

According to the present invention, compensation of the residual phaseerror can accurately be executed even when a wireless packet containingboth a portion to transmit data from a single antenna and a portion totransmit data from a plurality of antennas is received. According to awireless communication preamble signal system proposed in Jan Boer etal, “Backwards Compatibility”, IEEE 802.11-03/714r0, as shown in FIG.26, short preamble sequences x01 to be used for time synchronization,frequency synchronization, and AGC, long preamble sequences x02 forchannel response estimation, and a first signal field x03 includingfields representing the modulation method and length of the wirelesspacket are transmitted from one transmission antenna 101 a.Subsequently, a second signal field x04 to be used in IEEE 802.11n istransmitted. The number of transmission antennas to be multiplexed andthe multiplexing method are described in the second signal field. Next,long preamble sequences x05 for channel response estimation aresequentially transmitted from a transmission antenna 101 b. After thepreamble signals are transmitted, transmission data x08 and x09 aretransmitted simultaneously from the plurality of transmission antennas101 a and 101 b.

The wireless communication preamble signal shown in FIG. 26 is the sameas a wireless communication preamble signal based on the IEEE 802.11astandard shown in FIG. 27, which is based on transmission from thetransmission antenna 101 a from the short preamble x01 to the firstsignal field x03. With this arrangement, wireless reception apparatusbased on the IEEE 802.11a standard, which has received the preamblesignal shown in FIG. 26, can recognize the received packet as a wirelesspacket based on the IEEE 802.11a standard. Hence, the preamble signalshown in FIG. 26 allows IEEE 802.11n which simultaneously transmitsdifferent data from a plurality of antennas on one radio device tocoexist with the IEEE 802.11a standard which transmits data from asingle antenna.

In the IEEE 802.11a wireless packet shown in FIG. 27, pilot subcarriersare inserted from the SIGNAL field. On the basis of the pilotsubcarriers, compensation of the residual phase error can be executed.When the present invention is applied to the wireless packet shown inFIG. 26, pilot subcarriers are inserted in the SIGNAL field and SIGNAL2field. Pilot subcarriers are also located in the DATA portions x08 andx09. From the transmission data x08 and x09, the pilot subcarriersdescribed in the first to seventh embodiments of the present inventionare located. In the eighth embodiment, a case in which the pilotsubcarriers are transmitted by using the first embodiment will bedescribed.

Detailed control in receiving the wireless packet shown in FIG. 26 willbe described with reference to FIG. 28. FIG. 28 shows a receptionapparatus which receives the wireless packet shown in FIG. 26. FIG. 28is different from FIG. 5 in that the output from a decoder 207 is inputto a SIGNAL analysis unit 208, and a residual phase error detector 204is controlled on the basis of the result from the SIGNAL analysis unit208.

The receiver which has received the short preamble x01 detects the startof the long preamble sequence x02 by using an AGC and timesynchronization means (not shown) to detect an FFT window.Simultaneously, estimation and compensation of the frequency offset aredone. The receiver which has received the long preamble sequence x02measures the channel responses of all subcarriers by using known pilotsubcarriers. Especially, the channel response of a pilot subcarrier istransferred to the residual phase error detector 204. The aboveprocessing can be implemented by a known technique, and a descriptionthereof will be omitted.

Next, the SIGNAL field x03 is received. The SIGNAL field is subjected toFFT by FFT units 202 a and 202 b. The FFT output is input to aninterference suppression circuit. However, since the SIGNAL field isoutput from a single antenna, interference suppression need not beperformed. Hence, the processing executed by the interferencesuppression circuit is processing for multiplying a unit matrix orprocessing for weighting and combining the outputs from the FFT units202 a and 202 b to increase the Signal to Noise ratio. The output fromthe interference suppression circuit is input to the residual phaseerror detector 204.

FIG. 29 is a view showing the concept of processing executed by theresidual phase error detector according to the eighth embodiment. Assumethat the channel response when a long preamble which is transmitted fromthe transmission antenna 101 a and received by a reception antenna 201 ais received is Haa shown in FIG. 29. When a pilot subcarrier istransmitted by BPSK, the received signal point of the pilot subcarrierreceived by the data portion can take is a point (1) or (−1) shown inFIG. 29. On the basis of this point as a reference, a residual phaseerror is detected.

When the received point of the pilot subcarrier of the SIGNAL portion is“next symbol in single antenna transmission” shown in FIG. 29, theresidual phase error detector measures, as the residual phase error, aphase difference θ1 between the point (1) and the point of “next symbolin single antenna transmission”. The phase compensator corrects theresidual phase error. In receiving the SIGNAL2 portion, the residualphase error can be detected in the same way.

The demodulator 207 demodulates the SIGNAL2 portion and transfers thedecoding result to the SIGNAL analysis unit 208. The SIGNAL analysisunit 208 analyzes the second signal field, analyzes the number ofmultiplexed transmission antennas, and the information that there willbe a transmission from plurality of antennas after the long preamble.The SINGAL analysis unit 208 transfers the analyzed result to theresidual phase error detector 204.

Next, the receiver receives the long preamble from the transmissionantenna 101 b and measures the channel response from the transmissionantenna 101 b. A case in which the DATA portions x08 and x09 arereceived will be described next. In the following description, signalsfrom the two transmission antennas 101 a and 101 b are multiplexed inthe DATA portions, and the multiplexed pilot subcarriers have polaritiesrepresented by equations (4) and (5). As described above, the number ofmultiplexed transmission antennas can be recognized by the signal fromthe SIGNAL analysis unit 208.

Of the four pilot subcarriers, pilot subcarriers having the leftmostpolarities in equations (4) and (5) will be focused. The polarities ofpilot subcarriers transmitted by this frequency are Sa(1)=1 and Sb(1)=1.When the channel response from the transmission antenna 101 a to thereception antenna 201 a is Haa, and the channel response from thetransmission antenna 101 b to the reception antenna 201 a is Hba, thechannel response value of a pilot subcarrier received by the DATAportion is Haa+Hba shown in FIG. 29. The residual phase error detectorobtains a reference point by using the combined channel response valueHaa+Hba on the basis of the measured channel response value Haa+Hba andthe information from the SIGNAL analysis unit 208 and detects a shiftfrom the reference point.

More specifically, since the pilot subcarriers are transmitted by BPSKsignals modulated by a PN sequence, the points (1,1) and (−1,−1) shownin FIG. 29 are received. That is, when a single antenna is received, theresidual phase error can be detected on the basis of the points (1) and(−1) in FIG. 29. When the operation switches to transmission from theplurality of antennas, i.e., transmission of x08 or x09, the residualphase error must be detected on the basis of the points (1,1) and(−1,−1).

As described above, the operation of the residual phase error detector204 must be switched between single antenna transmission and multipleantenna transmission. In the eighth embodiment, this operation isimplemented by analyzing the second signal field x04 (SIGNAL2) shown inFIG. 26. In the second signal field, the number of signals multiplexedon the transmitting side or the multiplexing method is described. Hence,when the second signal field is analyzed the reference point of thechannel response value can be obtained. In addition, switching betweenthe reference point when a transmission signal from a single antenna isreceived and that when signals transmitted from a plurality of antennasare received can be done. For this reason, the reference signal pointcan appropriately be switched according to the case whether a signalfrom a single antenna is received or signals from a plurality ofantennas are received. In either case, the residual phase error canappropriately be detected and compensated when the wireless packet bothcontains the pilot subcarriers from a single antenna and the pilotsubcarriers from a polarity of antennas, i.e., the wireless packet bothcontains the packet based on IEEE802.11a and IEEE802.11n. In thisembodiment, x01, x02, x03, x04 are assumed to be transmitted from singleantenna, it can be transmitted from a plurality of antenna by usingcyclic delayed transmission diversity. The procedure of the phasecompensation is the same as written in this embodiment and it works.

Ninth Embodiment

In the above embodiments, OFDM signal transmission apparatuses whichtransmit different signals in correspondence with transmission antennas.However, the present invention can also be applied to an OFDM signaltransmission apparatus using a transmission method of transmittingdifferent signals in correspondence with a plurality of transmissionbeams.

FIG. 31 shows an OFDM transmission apparatus according to the ninthembodiment of the present invention, which transmits different signalsin correspondence with a plurality of transmission beams. Referring toFIG. 31, an encoder 102, serial-parallel converter 103, modulators 104 aand 104 b, serial-parallel converters 105 a and 105 b, and pilotsubcarrier insertion unit 106 are the same as in the first to eighthembodiments, and a description thereof will be omitted.

According to the ninth embodiment, subcarriers obtained by insertingpilot subcarriers between data subcarriers by the pilot subcarrierinsertion unit 106 are input to a beam forming device 108. The beamforming device 108 weights and combines the outputs from theserial-parallel converter 105 a and the outputs from the serial-parallelconverter 105 b and outputs the signals to IFFT units 107 a and 107 b.The outputs after IFFT are output from transmission antennas 101 a and101 b.

The beam forming device 108 is a device which executes processing forforming (beam forming) a plurality of transmission beams. The beamforming device 108 can be implemented by using a known technique.Referring to FIG. 31, the beam forming device 108 is inserted before theIFFT units 107 a and 107 b. The beam forming device 108 may be insertedafter the IFFT units 107 a and 107 b.

Known beam forming schemes are roughly classified into two shames. Inthe first beam forming mode, the channel response between a radiotransmission apparatus and a radio reception apparatus is completelyknown. A beam forming weight is calculated in accordance with thechannel response in order to steer the beam toward the receptionapparatus. In the second beam forming mode, the channel response is notcompletely known and a predetermined beam forming weight is used.Therefore, the beam may not be steered toward the reception apparatus.John Ketchum et al have described the Eigenvector steering (ES) schemeas one of the first beam forming schemes and the Spatial spreading (SS)scheme as one of the second beam forming schemes in reference“ftp://ieee:wireless@ftp.802wirelessworld.com/11/04/11-04-0870-00-000n-802-11-ht-system-description-and-operating-principles.doc.”According to a description in the reference by John Ketchum et al, thediversity effect can be obtained by using beam forming.

The beam forming device 108 has, e.g., the ES scheme and SS scheme tocope with the first beam forming scheme and second beam forming scheme.For data subcarriers, beam forming can effectively be done by usingeither the ES scheme or SS scheme, as described by John Ketchum et al.On the other hand, for pilot subcarriers, beam forming by the methodinstructed by John Ketchum et al is not preferable. This is because thepilot subcarriers are known information on the receiving side and mustalways correctly be received on the receiving side.

Pilot subcarrier forming methods using the ES scheme and SS scheme willbe described next.

In the ES scheme, the channel response between a radio transmissionapparatus and a radio reception apparatus is measured, and thetransmission beam is directed in the direction of the radio receptionapparatus, as described in p. 10 of the reference by John Ketchum et al.Since the pilot signal arrives at the radio reception apparatusregardless of pilot subcarriers generated by the pilot subcarrierinsertion unit 106, no dead zone is formed. Hence when a beam should beformed by using the ES scheme, the same setting as in the first toeighth embodiment is done for the polarities of pilot subcarriers. Onthe other hand, the SS scheme is a beam forming scheme when the channelresponse is not completely known and the predetermined value is used asa beam forming weight. Hence, it cannot guarantee that the pilotsubcarriers should always arrive at the radio reception apparatus.

Pilot subcarriers when the number of transmission antennas is 4 isrepresented by a matrix P.

$P = \begin{bmatrix}1 & 1 & 1 & {- 1} \\1 & {- j} & {- 1} & {- j} \\1 & {- 1} & 1 & 1 \\1 & j & {- 1} & j\end{bmatrix}$

The number of columns of the matrix P equals the number of pilotsubcarriers on the frequency axis. The first row of the matrix Prepresents the polarity of the first pilot subcarrier transmitted fromeach antenna. The number of rows of the matrix P equals the number ofinput ports of the beam forming device 108. That is, the matrix Prepresents FIG. 11B. A beam forming matrix Q using a Walsh matrixdescribed in the reference by John Ketchum et al is given by

$Q = \begin{bmatrix}1 & 1 & 1 & 1 \\1 & {- 1} & 1 & {- 1} \\1 & 1 & {- 1} & {- 1} \\1 & {- 1} & {- 1} & 1\end{bmatrix}$

The number of columns of the matrix Q equals the number of input portsof the beam forming device 108, i.e., the number of transmission beams.The number of rows of the matrix P equals the number of transmissionantennas. The i-th row of the matrix Q represents the weightcorresponding to the i-th transmission beam (in this case, i=1, 2, 3,4). The beam forming device 108 multiplies the matrix P by the matrix Qto obtain a matrix QP given by

${QP} = \begin{bmatrix}4 & 0 & 0 & 0 \\0 & 0 & 4 & 0 \\0 & {2 - {2j}} & 0 & {{- 2} - {2j}} \\0 & {2 + {2j}} & 0 & {{- 2} + {2j}}\end{bmatrix}$

The number of columns of the matrix QP equals the number of pilotsubcarriers on the frequency axis. The number of rows of the matrix QPequals the number of transmission antennas. For example, the firstcolumn of the matrix QP will be focused. This corresponds totransmitting a pilot with a power of “16” (i.e., amplitude is 4) fromonly the first transmission antenna. The second column of the matrix QPwill be focused. This corresponds to transmitting pilot signals with apower of “8” (i.e., amplitude is root 8) from only the third and fourthtransmission antennas. As described above, especially for the firstpilot subcarrier, it will be transmitted with high power only from thefirst transmission antenna. Regarding the second pilot subcarrier, apilot signal is transmitted only from the third and fourth transmissionantennas. As a result, the transmission power varies in the frequencyband of the OFDM signal. As a result, the problems pointed out in theprior art, i.e., composite triple beat and an increase in dynamic rangeof the transmission signal occur.

More specifically, when beam forming based on the SS scheme is appliedto pilot subcarriers, the orthogonality of the pilot subcarriers isdamaged. For certain pilot subcarriers, some transmission antennastransmit no pilot signals. Hence, the space diversity effect decreases.

In the ninth embodiment, when the beam forming device 108 uses beamforming based on the SS scheme, the SS scheme is used for datasubcarriers, as described in the reference by John Ketchum et al.However, no beam forming is executed for pilot subcarriers. Morespecifically, for pilot subcarriers, the matrix Q should be the identifymatrix Q′ as follows.

$Q^{\prime} = \begin{bmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{bmatrix}$

The first row of the matrix Q′ represents the weight corresponding tothe i-th transmission beam (in this case, i=1, 2, 3, 4). In this case,since the matrix P directly appears in the matrix Q′P, pilot signals aretransmitted by pilot subcarriers which are orthogonal to each other onthe frequency axis and spatial axis.

When SS scheme is used, cyclic delayed diversity (CDD) is applied to thetransmission signal from each antenna, respectively. Specifically, thetransmission signal from the first transmission antenna is generated tothe transmission signal from the second transmission antenna by 50 nseccyclic shifting. Since the CDD scheme is described in the reference byJohn Ketchum et al, explanation is omitted.

As described above, in the ninth embodiment, when beam forming isexecuted in accordance with the second beam forming mode and, forexample, the SS scheme without using channel response information, pilotsignals are transmitted without applying beam forming to pilotsubcarriers. As a result, pilot signal transmission from only a singleantenna can be prevented, and any increase in dynamic range of thetransmission signal can be prevented. In addition, any dead zone on thereceiving side can be prevented by the space diversity effect.

In the above-mentioned description, beam forming not applied to thepilot subcarrier in the second beam formation mode (multiplied by theidentify matrix). However, for example, when beam forming matrix Q′which multiplied to matrix P with the beam formation device 107 isassumed given by

$Q^{\prime} = \begin{bmatrix}4 & 0 & 0 & 0 \\0 & 0 & {2 + {2j}} & {{- 2} + {2j}} \\0 & 4 & 0 & 0 \\0 & 0 & {{- 2} + {2j}} & {2 + {2j}}\end{bmatrix}$Matrix Q′P given by

${Q^{\prime}P} = \begin{bmatrix}1 & 1 & 1 & {- 1} \\1 & {- 1} & 1 & 1 \\1 & 1 & {- 1} & 1 \\1 & {- 1} & {- 1} & 1\end{bmatrix}$

From each antenna, the pilot subcarrier which orthogonal to each otheron the frequency axis and the space axis is transmitted so that matrixQ′P may show. This is a polarity pattern of the pilot subcarrier shownin FIG. 11A. As a result, it is avoided that the pilot signal istransmitted only from single antenna and the dynamic range of atransmission signal is expanded.

Thus, in the second mode that is the mode which is not based on thechannel response but forms the transmission beam, by making the wait ofthe transmission beam to the data subcarrier, and the wait of thetransmission beam to the pilot subcarrier into a different value. It isavoided that a pilot signal inclines and is transmitted from singleantenna, and since the pilot subcarrier which are orthogonal to eachother on the frequency axis and the space axis is transmitted from eachantenna, it is avoidable that the dead zone is generated in thereceiving side.

In this embodiment, the polarity pattern of pilot subcarriers is fixedin a wireless packet for the descriptive convenience. Even when thepolarity pattern is changed in a wireless packet, i.e., for each OFDMsymbol, the effect of the present invention can be obtained. As shown inFIG. 12, the radio transmitter stores a plurality of polarity patternsin the ROM. A counter which counts the number of OFDM symbols isarranged in place of the radio packet counter. The polarity pattern ofpilot subcarriers may be changed by selecting one of the plurality ofpolarity patterns for each antenna in accordance with the counter.

10th Embodiment

The 10th embodiment of the present invention will be described next.This embodiment is different from the first to ninth embodiments in thatthe polarity of a pilot subcarrier changes for each OFDM symbol, and thefrequency of the pilot subcarrier, i.e., the position of the pilotsubcarrier periodically changes. When the position of a pilot subcarrierperiodically changes, the channels of all subcarriers can be estimatedduring the data portions.

FIGS. 32 to 35 show a pilot subcarrier location according to thisembodiment when the number of transmission antennas is four. FIG. 32shows the pilot subcarrier location in an OFDM signal transmitted fromthe first transmission antenna. Similarly, FIGS. 33, 34, and 35 showsthe pilot subcarrier locations in OFDM signals transmitted from thesecond, third, and fourth transmission antennas, respectively. Referringto FIGS. 32 to 35, the abscissa represents subcarrier numbers (−26 to 26in this example), i.e., the frequencies (positions) of subcarriers. Theordinate represents OFDM symbol numbers (DATA1 to DATA12 in thisexample). Pilot subcarriers are located in cells except blank cells.Symbols in cells indicate the polarities of the pilot subcarriers. No PNsequence is illustrated in FIGS. 32 to 35. A description of datasubcarriers is not illustrated, either.

In this embodiment, the pilot subcarriers are orthogonal to each otherin a plurality of unit times between transmission antennas. In theexample shown in FIGS. 32 to 35, one OFDM symbol time is defined as aunit time. The pilot subcarriers are orthogonal to each other betweenthe four transmission antennas in a 4-OFDM-symbol interval. Thefrequencies (positions) of pilot subcarriers change at a periodcorresponding to an integer multiple of the plurality of unit times and,for example, at a period corresponding to four OFDM symbols.

By using the same description as in equation (9), the polarity of eachpilot subcarrier shown in FIGS. 32 to 35 is given by

$\begin{matrix}{{Sp}_{{{{pos}{({i,t})}}k},t} = {{\exp\left( {\frac{j\pi}{2}\left( {k - 1} \right){\mathbb{i}}} \right)} \times {P_{rot}\left( {i,t} \right)}}} & (27)\end{matrix}$where Sp_(pos(i,t),k,t) indicates that the (Sp_(pos(i,t)))th subcarrieris used as a pilot subcarrier, k is the transmission antenna number, andt is the OFDM symbol number. In equation (27), the right-hand member ofequation (9) is multiplied by polarity data P_(rot)(i,t) given by

$\begin{matrix}{{P_{rot}\left( {i,t} \right)} = {\exp\left( {\frac{j\pi}{2}\left( {t - 1} \right){\mathbb{i}}} \right)}} & (28)\end{matrix}$where P_(pos)(i,t) is a conversion function to convert the subcarriernumber from the pilot subcarrier number i and OFDM symbol number t. Morespecifically, P_(pos)(i,t) can be expressed by

$\begin{matrix}{{P_{pos}\left( {i,t} \right)} = \left\{ {{{\begin{matrix}{- 20} & {{{for}\mspace{14mu} i} = 0} \\{- 7} & {{{for}\mspace{14mu} i} = 1} \\{+ 7} & {{{for}\mspace{14mu} i} = 2} \\{+ 20} & {{{for}\mspace{14mu} i} = 3}\end{matrix}\mspace{14mu} 1} \leq t \leq {4{P_{pos}\left( {i,t} \right)}}} = \left\{ {{{\begin{matrix}{- 17} & {{{for}\mspace{14mu} i} = 0} \\{- 4} & {{{for}\mspace{14mu} i} = 1} \\{+ 10} & {{{for}\mspace{14mu} i} = 2} \\{+ 23} & {{{for}\mspace{14mu} i} = 3}\end{matrix}\mspace{14mu} 5} \leq t \leq {8{P_{pos}\left( {i,t} \right)}}} = \left\{ {{\begin{matrix}{- 14} & {{{for}\mspace{14mu} i} = 0} \\{- 1} & {{{for}\mspace{14mu} i} = 1} \\{+ 13} & {{{for}\mspace{14mu} i} = 2} \\{+ 26} & {{{for}\mspace{14mu} i} = 3}\end{matrix}\mspace{14mu} 9} \leq t \leq 12} \right.} \right.} \right.} & (29)\end{matrix}$

FIG. 36 shows the value of the polarity data P_(rot)(i,t) obtained bysubstituting the transmission antenna number k and OFDM symbol number tinto equation (28). As is apparent from FIG. 36, equation (28) has aperiodicity for the OFDM symbol number t. In this embodiment, the periodis 4-OFDM-symbol interval. More specifically, the polarity dataP_(rot)(i,t) has the same values when t=1 and t=5.

As is apparent from equation (29), the position (frequency) of the pilotsubcarrier changes in accordance with the period of equation (28). Forexample, in subcarriers transmitted from the first to fourthtransmission antennas, first, pilot subcarriers are located at positionscorresponding to the subcarrier numbers of −20, −7, +7, and +20 in thefirst 4-OFDM-symbol interval (interval from DATA1 to DATA4) where t=1 to4.

For example, the pilot subcarriers located at the subcarrier number of−20 will be focused. The polarities of pilot subcarriers transmittedfrom the first transmission antenna are “1, 1, 1, 1”. The polarities ofpilot subcarriers transmitted from the second transmission antenna are“1, j, −1, −j”. The polarities of pilot subcarriers transmitted from thethird transmission antenna are “1, −1, 1, −1”. The polarities of pilotsubcarriers transmitted from the fourth transmission antenna are “1, −j,−1, j”. The pilot subcarriers transmitted from the first to fourthtransmission antennas are orthogonal to each other in the 4-OFDM-symbolinterval. This also applies to the pilot subcarriers located at theremaining subcarrier numbers of −7, +7, and +20. The pilot subcarriersare orthogonal to each other in the 4-OFDM-symbol interval.

In the next 4-OFDM-symbol interval (interval from DATA5 to DATA8) wheret=5 to 8, the pilot subcarriers at the subcarrier numbers of −17, −4,+10, and +23 are transmitted. The polarities of the pilot subcarrierstransmitted from the first to fourth transmission antennas are the sameas in the first 4-OFDM-symbol interval where t=1 to 4 so that the pilotsubcarriers are orthogonal to each other.

As shown in FIGS. 32 to 35 and equation (27), the pilot subcarriers areorthogonal to each other even on the frequency axis. For example, thepilot subcarriers shown in FIG. 33, which are transmitted from thesecond transmission antenna, will be focused. In the first 4-OFDM-symbolinterval (interval from DATA1 to DATA4) where t=1 to 4, the polaritiesof the pilot subcarriers located at the subcarrier number −20 are “1, j,−1, −j”. The polarities of the pilot subcarriers located at thesubcarrier number −7 are “−j, 1, j, −1”. The polarities of the pilotsubcarriers located at the subcarrier number +7 are “−1, −j, 1, j”. Thepolarities of the pilot subcarriers located at the subcarrier number +20are “−j, 1, j, −1”. The pilot subcarriers are orthogonal to each other.

The period of equation (28) need not always equal the change period ofthe pilot subcarrier position indicated by equation (29). The period ofequation (29) only needs to be an integer multiple of the period ofequation (28). Equation (29) is generated from Fourier coefficients.They only need to be sequences orthogonal with respect to the time axist. FIG. 37 shows another example of polarity data P_(pos)(k) when realnumbers are used.

A pilot subcarrier insertion unit 106 according to this embodiment willbe described next with reference to FIG. 38. The overall arrangement ofthe OFDM signal transmission apparatus including the pilot subcarrierinsertion unit 106 is the same as in FIG. 3, and a description thereofwill be omitted. In the pilot subcarrier insertion unit 106 shown inFIG. 38, transmission data from serial-parallel converters 105 a and 105b in FIG. 3 are input to subcarrier locating devices 126 a and 126 b,respectively.

First pilot subcarriers generated by causing multiplying units 111 a to111 d to obtain the products between a PN sequence from a PN sequencegenerator 110 and polarity data Sa(j) of the first pilot subcarriers,which are stored in a ROM 121 a, are input to the subcarrier locatingdevice 126 a. Similarly, second pilot subcarriers generated by causingmultiplying units 112 a to 112 d to obtain the products between the PNsequence from the PN sequence generator 110 and polarity data Sb(j) ofthe second pilot subcarriers, which are stored in a ROM 121 b, are inputto the subcarrier locating device 126 b. P_(rot)(i,t) represented byequations (27) to (29) are stored in the ROMs 121 a and 121 b as thepolarity data Sa(j) and Sb(j).

The processing for changing the polarities or positions of pilotsubcarriers for each OFDM symbol is executed in the following manner. Intransmitting DATA1, a counter 131 counts a clock signal 130 of the OFDMsymbol period and recognizes that it is time to transmit DATA1. Thecounter 131 outputs, to a subcarrier locating controller 132 and pilotpattern controller 133, instructions to control the subcarrier locatingand pilot pattern for DATA1. In transmitting DATA2, the counter 131recognizes by the clock signal 130 that it is time to transmit DATA2.The counter 131 outputs, to the subcarrier locating controller 132 andpilot pattern controller 133, instructions to control the subcarrierlocating and pilot pattern for DATA2. More specifically, the pilotpattern controller 133 reads out values according to equations (27) and(28) from the ROMs 121 a and 121 b. The subcarrier locating controller132 inserts pilot subcarriers at positions according to equation (29).In this way, the pilot subcarriers shown in FIGS. 32 to 35 are formed.

An OFDM signal reception apparatus having a function of receiving thepilot subcarriers indicated by equation (27) and detecting a residualphase error will be described next with reference to FIG. 39.

An OFDM signal received by a reception antenna 201 a is input to an FFTunit 202 a through a radio reception unit (not shown) and divided intosubcarrier signals by Fourier transform. An OFDM signal received by areception antenna 201 b is also subjected to Fourier transform by an FFTunit 202 b and divided into subcarrier signals by Fourier transform.

Of the subcarriers output from the FFT units 202 a and 202 b, datasubcarriers are input to an interference suppression circuit 203, andpilot subcarriers are input to a residual phase error detector 204 andchannel variation detector 210. The residual phase error detected by theresidual phase error detector 204 is compensated by phase compensators205 a and 205 b. The channel variation detected by the channel variationdetector 210 is compensated by channel variation compensators 211 a and211 b. Elements except the residual phase error detector 204, channelvariation detector 210, and channel variation compensators 211 a and 211b are the same as in the OFDM reception apparatus shown in FIG. 5, and adescription thereof will be omitted.

In this embodiment, the phase compensator 204 and channel compensators211 a and 211 b are independently arranged. These devices can beregarded as devices to compensate the distortion of the signals. Hence,the phase compensator 204 and channel compensators 211 a and 211 b canbe implemented by a single compensator. In this embodiment, the phasecompensator 204 and channel compensators 211 a and 211 b are arranged onthe output side of the interference suppression circuit 203. Instead,they can also be arranged on the input side.

The residual phase error detector 204 detects the residual phase errorby using pilot subcarriers. The polarity data of pilot subcarriers ineach OFDM symbol and the PN sequence to be multiplied by the polaritydata are known in the OFDM signal reception apparatus. Hence, theresidual phase error detector 204 can detect the residual phase error byusing one of the detection principle shown in FIG. 6 described in thefirst embodiment, the detection principle shown in FIG. 18 described inthe fourth embodiment, the detection principle shown in FIGS. 23A and23B described in the sixth embodiment, and the detection principle shownin FIG. 29 described in the eighth embodiment.

Propagation path estimation using pilot subcarriers will be describednext. For example, the 20th pilot subcarrier will be described. Let Haabe the channel response value from the transmission antenna 101 a to thereception antenna 201 a, Hba be the channel response value from thetransmission antenna 101 b to the reception antenna 201 a, Hca be thechannel response value from a transmission antenna 101 c (not shown) tothe reception antenna 201 a, and Hda be the channel response value froma transmission antenna 101 d (not shown) to the reception antenna 201 a.

In this case, when noise components are moved, signals receives in theintervals of DATA1, DATA2, DATA3, and DATA4 can be given byr _(DATA1) =H _(aa) +H _(ba) +H _(ca) +H _(da)  (30)r _(DATA2)=(H _(aa) +jH _(ba) −H _(ca) −jH _(da))exp(jθ)  (31)r _(DATA3)=(H _(aa) −H _(ba) +H _(ca) −H _(da))exp(j2θ)  (32)r _(DATA4)=(H _(aa) −jH _(ba) −H _(ca) +jH _(da))exp(j3θ)  (33)These are signals before the residual phase error is removed, andexp(jnθ)(n=1, 2, 3) represents the residual phase error. The channelestimation value of Haa can be obtained by executing processing given by

$\begin{matrix}\begin{matrix}{{\hat{H}}_{aa} = {r_{{DATA}\; 1} + r_{{DATA}\; 2} + r_{{DATA}\; 3} + r_{{DATA}\; 4}}} \\{= {{H_{aa}\left( {1 + {\exp({j\theta})} + {\exp\left( {j\; 2\theta} \right)} + {\exp\left( {j\; 3\theta} \right)}} \right)} +}} \\{{H_{ba}\left( {1 + {j\;{\exp({j\theta})}} - {\exp\left( {j\; 2\theta} \right)} - {j\;{\exp({j3\theta})}}} \right)} +} \\{{H_{ca}\left( {1 - {\exp\left( {j\;\theta} \right)} + {\exp\left( {j\; 2\theta} \right)} + {\exp({j3\theta})}} \right)} +} \\{H_{da}\left( {1 - {j\;{\exp({j\theta})}} - {\exp\left( {j\; 2\theta} \right)} + {j\;{\exp({j3\theta})}}} \right)}\end{matrix} & (34)\end{matrix}$

Since the phase difference between several symbols represented by theresidual phase error exp(jnθ)(n=1, 2, 3) can be assumed to besufficiently close to 1, the estimation value of Haa can be obtained byequation (34). This is because the pilot subcarriers are transmitted ata period (4-OFDM-symbol interval in this embodiment) at which the pilotsubcarriers are orthogonal to each other, as shown in FIGS. 32 to 35.

The estimation value of Hba can also be obtained byĤ _(ba) =r _(DATA1) −jr _(DATA2) −r _(DATA3) +jr _(DATA4)  (35)

Generally, when calculation using the complex conjugate of the sequencesshown in FIGS. 32 to 35 as a coefficient is executed, the channelresponse value corresponding to each transmission antenna can beobtained. For example, as described in the eighth embodiment, when awireless packet structure as shown in FIG. 26 is used, the channelresponses of all subcarriers can be estimated by the long preambleportion at the start of the packet. However, the channel response canvary in the packet in accordance with the ambient variation. Especially,when the length of the wireless packet is more than the variation of thechannel, the channel estimation value obtained by the long preambleportion may be different from that estimated by the DATA1 to DATA4portions. Even in this case, the channel variation compensators 211 aand 211 b can compensate for the variation by making the pilotsubcarriers be orthogonal to each other in a plurality of unit timesbetween the plurality of transmission antennas and changing thefrequencies (positions) of the pilot subcarriers at a periodcorresponding to an integer multiple of the unit time.

In the above description, the channel variation value is estimated byusing signals before residual phase error compensation. The channelvariation can also be estimated by using signals after residual phaseerror compensation. In this case, since the residual phase errorcomponents in equations (30) to (33) can be neglected, the channelvariation value can accurately be obtained.

FIG. 40 shows the arrangement of an OFDM reception apparatus whichestimates the channel variation by using signals after residual phaseerror compensation. Referring to FIG. 40, the outputs from the FFT units202 a and 202 b are transferred to the residual phase error detector 204and phase compensators 205 a and 205 b. By using the signals which haveundergone phase compensation by the phase compensators 205 a and 205 b,the channel variation detector 210 detects the variation in channelresponse.

As described above, in this embodiment, the positions of pilotsubcarriers are changed for each orthogonal interval of the pilotsubcarriers. The orthogonal interval indicates an interval where thepilot subcarriers are orthogonal to each other between the transmissionantennas and 4-OFDM-symbol interval in the example shown in FIGS. 32 to35. When the processing expressed by equation (34) is executed by usingthe reception signals in this orthogonal interval the channel estimationvalues from the transmission antennas can be obtained. After pilotsubcarriers are transmitted in one orthogonal interval, othersubcarriers are used as pilot subcarriers. Accordingly, the channelestimation values corresponding to all subcarriers can be obtained.

In the above description, since the number of transmission antennas isfour, sequences (e.g., FIGS. 32 to 35) in which the orthogonal period ofpilot subcarriers is 4-OFDM-symbol interval are used. When the number oftransmission antennas is three or two, sequences with a shorterorthogonal period can be used. In this embodiment, orthogonalization isimplemented by changing the polarities of pilot subcarriers. Instead,channel estimation can also be done by implementing orthogonalization bychanging the type of PN sequence for each transmission antenna. Morespecifically, as described in the fourth embodiment, a PN sequence isprepared for each transmission antenna to make pilot subcarriersorthogonal to each other on the time axis, thereby executing channelestimation of the 10th embodiment.

FIG. 41 shows the arrangement of the pilot subcarrier insertion unit 106when pilot subcarriers are made orthogonal on the time axis by changingthe PN sequence. This pilot subcarrier insertion unit is different fromthat shown in FIG. 38 in the following point. In the pilot subcarrierinsertion unit shown in FIG. 38, the polarities of pilot subcarriers arechanged for each OFDM symbol, i.e., each DATA. In the pilot subcarrierinsertion unit shown in FIG. 41, PN sequence generators 110 a and 110 bare prepared for the respective transmission antennas.

As described above, according to the 10th embodiment, pilot subcarriersare orthogonal to each other in a plurality of unit times between aplurality of transmission antennas. In addition, the frequencies(positions) of the pilot subcarriers are changed at a periodcorresponding to an integer multiple of the unit time. Furthermore, thepilot subcarriers are orthogonal to each other even on the frequencyaxis. Hence, directional patterns formed by the transmission antennas intransmitting pilot subcarriers can variously be changed.

As in the first to ninth embodiments, the dead zone where the receivedpowers of all four pilot subcarriers decrease at the same time, can bereduced. The area where high-quality reception is possible can bewidened. Simultaneously, any composite triple beat at the transmittercan be prevented. On the receiving side, the channel estimation valuecorresponding to each transmission antenna can easily be obtained duringthe data portions.

In this embodiment, subcarriers to transmit pilot signals change foreach time. Even when the subcarriers to transmit pilot signals decreasedue to fading to make it difficult to measure the residual phase error,pilot signals are transmitted from other subcarriers in the next OFDMsymbol. Hence, the pilot signals can accurately be received at a highprobability during the data portions.

According to the present invention, even when the received power of apilot subcarrier is low, the received power of another pilot subcarrierbecomes high at a high probability. Hence, the dead zone where thereception powers of all pilot subcarriers decrease simultaneously can bereduced, and the area where high-quality reception is possible widens.In addition, since the transmission powers of pilot subcarriers from thetransmission antennas are the same as those of data subcarrier, anycomposite triple beat can be avoided. In addition, the input dynamicrange of the D/A converter need not particularly be wide.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A communication device for transmitting anorthogonal frequency division multiplexing (OFDM) signal, comprising: afirst antenna; a second antenna; a generator configured to generate datasignals and pilot signals to be transmitted via a plurality ofsubcarriers, the pilot signals to be transmitted including first pilotsignals and second pilot signals, the first pilot signals to betransmitted via the first antenna having first polarities, the secondpilot signals to be transmitted via the second antenna having secondpolarities, the first polarities being different from the secondpolarities; a beamformer configured to form first beams for transmittingthe data signals by using first weights and to form second beams fortransmitting the pilot signals by using second weights, the secondweights being different from the first weights, wherein the data signalsand the pilot signals are to be transmitted via at least one of thefirst antenna and the second antenna.
 2. The communication deviceaccording to claim 1, wherein the first weights are determined by usinga value of a channel response between the communication device andanother communication device which is a receiver of the OFDM signal. 3.The communication device according to claim 1, wherein the secondweights are determined by not using a value of a channel responsebetween the communication device and another communication device whichis a receiver of the OFDM signal.
 4. The communication device accordingto claim 1, wherein the first pilot signals are orthogonalized with thesecond pilot signals.
 5. The communication device according to claim 1,further comprising: a first memory configured to store first polaritydata; a second memory configured to store second polarity data; a firstmultiplier configured to generate the first pilot signals by multiplyinga first sequence by the first polarity data; and a second multiplierconfigured to generate the second pilot signals by multiplying a secondsequence by the second polarity data.
 6. The communication deviceaccording to claim 4, wherein the second weights are unit matrixes.
 7. Acommunication device for transmitting an orthogonal frequency divisionmultiplexing (OFDM) signal via a plurality of transmission antennas,comprising: a first antenna; a second antenna; a generator configured togenerate data signals and pilot signals to be transmitted via aplurality of subcarriers, the pilot signals to be transmitted includingfirst pilot signals and second pilot signals, the first pilot signals tobe transmitted via the first antenna having first polarities, the secondpilot signals to be transmitted via the second antenna having secondpolarities, the first polarities being different from the secondpolarities; a beamformer configured to form first beams for transmittingthe data signals by using first weights, wherein the beamformer isconfigured not to perform a beamforming process on the pilot signals,and the data signals and the pilot signals are to be transmitted via atleast one of the plurality of antennas.
 8. The communication deviceaccording to claim 7, wherein the first weights are determined by usinga value of a channel response between the communication device andanother communication device which is a receiver of the OFDM signal. 9.The communication device according to claim 7, wherein the first pilotsignals are orthogonalized with the second pilot signals.
 10. Thecommunication device according to claim 7, further comprising: a firstmemory configured to store first polarity data; a second memoryconfigured to store second polarity data; a first multiplier configuredto generate the first pilot signals by multiplying a first sequence bythe first polarity data; and a second multiplier configured to generatethe second pilot signals by multiplying a second sequence by the secondpolarity data.
 11. A communication method for transmitting an orthogonalfrequency division multiplexing (OFDM) signal, comprising: generatingdata signals and pilot signals to be transmitted via a plurality ofsubcarriers, the pilot signals to be transmitted including first pilotsignals and second pilot signals, the first pilot signals to betransmitted via a first antenna having first polarities, the secondpilot signals to be transmitted via a second antenna having secondpolarities, the first polarities being different from the secondpolarities; forming first beams for transmitting the data signals byusing first weights and forming second beams for transmitting the pilotsignals by using second weights, the second weights being different fromthe first weights, wherein the data signals and the pilot signals are tobe transmitted via at least one of the first antenna and the secondantenna.
 12. A communication method for transmitting an orthogonalfrequency division multiplexing (OFDM) signal via a plurality oftransmission antennas, comprising: generating data signals and pilotsignals to be transmitted via a plurality of subcarriers, the pilotsignals to be transmitted including first pilot signals and second pilotsignals, the first pilot signals to be transmitted via the first antennahaving first polarities, the second pilot signals to be transmitted viathe second antenna having second polarities, the first polarities beingdifferent from the second polarities; forming first beams fortransmitting the data signals by using first weights, wherein theforming the first beams for transmitting the data signals does notinclude performing a beamforming process on the pilot signals, and thedata signals and the pilot signals are to be transmitted via at leastone of the plurality of antennas.